The electrolytic oxidation of the Fe.sup.2+ ions in an exhausted pickling solution in order to restore the necessary concentration of Fe.sup.3+ ions has been already disclosed by U.S. Pat. No. 3,622,478 for the treatment of a pickling solution based on H.sub.2 SO.sub.4 and Fe.sup.3+ ions introduced in starting solution as Ferric sulphate. The treatment is carried out in an electrolytic cell without separation between the catholyte and the anolyte (cell without separating diaphragm).
In FR 2.341.669 it is disclosed the electrolytic oxidation of Fe.sup.2+ ions in an exhausted pickling solution based on HCl and Fe chlorides, in order to restore the necessary concentration of Fe.sup.3+ ions. The treatment is carried out in a cell provided by separating diaphragm.
The above mentioned process for electrolytic oxidation of Fe.sup.2+ ions are relevant to pickling solutions based on H.sub.2 SO.sub.4 or HCl and not containing HF or F.sup.- anions.
The presence of HF and F.sup.- anions in the pickling solution involves the formation of fluorinated complexes of trivalent iron and consequently the properties and behaviour of the solution is not equivalent to those of pickling solutions not containing HF (or F.sup.- anions).
Consequently the above prior art electrolytic oxidation methods cannot be considered obviously applicable to HF containing pickling solutions.